I'm currently reading through
Fire On The Velvet Horizon which is a new book of about 100 monsters by Patrick Stuart and Scrap Princess. It's pretty damn amazing... at least for the sort of monsters I enjoy.
It's rules agnostic and few of the creatures are the sort of thing you just meet in a dark tunnel, hack to death and steal their treasure. Each one takes up a page and has the potential to be the center of an entire adventure/campaign.
Like, the very first creature in the book...
It does not make direct attacks and can't be taken out by them... it's a creature of absolute law and hate that situates itself in an area and will use the rules and regulations of that place to take out everyone it can using the legal system. Attacking it would be bad manners... so you can't, because in its presence all social laws/manners become physical laws.
In order to defeat it you would either have to create an atmosphere of total chaos, dismantle any existing civilization... OR... somehow frame the creature for a capital offense and it would then be obligated by its nature to obey the sentence.
Most all the ones I've read so far have been mind-benders... which means I'd probably be inclined to use them sparingly, these are the sorts of critters you'd randomly sprinkle around in a dungeon.
The caveat is that the book is full color and priced accordingly... each page is a hand-crafted piece of art that reads like a crazed ransom note from a madman. There are no nods to the usual accessible sort of layout we get in most RPG supplements. Reading the thing is an experience unto itself... including the small type that flows deep into the center of the book.
As it is, even if I weren't into RPGs I'd love this book as a work of horror fiction/illustration.